Python refactor skeleton

[yochem]

This is the first step of the work done in #144 and simply wraps the SpiDev class defined in the C module.

Please note that although this PR already looks quite large, most of the code looks like this:

def open_path(self, path: str) -> None:
    self._cmod.open_path(path)

Where the C module method is simply wrapped.

TODO:

• Remove the module definition stuff from the C module
• Test building of the C module

Next steps (after this PR):

• Add Python goodies (__str__ and friends)
• Proper subclass of one of the I/O base classes
• Add the new interface methods, soft deprecating old ones
• Docstrings

[yochem]

The skeleton is ready for review. It tries to do as few as possible by simply wrapping all properties and methods of the original SpiDev class.

• Original (C module's) SpiDev class is renamed to spidev._spidev._SpiDev
• Implementation moved to spidev/_spidev.c
• New wrapper is spidev.SpiDev and defined in spidev/_spi.py
• Both are 100% type annotated (the C module using a .pyi stub)

I was wondering if you want to merge this directly into master, or create a new branch where we'll continue lifting stuff from C to the wrapper and add the pythonic interface? Mostly with the suggestions from #144.